This is a study of the epidemiology and etiology of adolescent loneliness in a national sample of youth. The descriptive objectives include: (1) examining the multidimensionality of loneliness as experienced by American youth so as to establish the multidimensionality of loneliness as experienced by American youth so as to establish the nature and meaning of this experience; (2) establishing the incidence of particular forms or dimensions of loneliness, both for the general adolescent population and for specific age, sex, class, and ethnic subpopulations; (3) describing patterns for change in the degree and form of loneliness across time for different age, class, race, and sex groups; and (4) describing the form and frequency of certain responses to loneliness. The explanatory objectives involve a test of a general theorectical model specifying precipitation and predisposing factors in the emergence of loneliness. Basically, loneliness is viewed as the result of (1) weak or attenuated internal bonds; (2) weak or attenuated external bonds (deficits in social relationships); and (3) precipitating factors associated with developmental changes experienced during adolescence. The data include three national surveys with a youth panel (N equals 17349) completed in 1977, 1978, and 1979. Planned analyses include a number of multivariate taxonomic methods, comparative factor analyses, multiple regression, and age-cohort analyses employing t-tests and ANOVA techniques.